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Commitbc9236b

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committed
Revise _bt_binsrch() so that its binary search loop takes
care of equal-key cases, eliminating bt_firsteq(). The linear searchformerly done by bt_firsteq() took a lot of time in the case where manyequal keys appear on the same page.
1 parent9679cb3 commitbc9236b

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1 file changed

+85
-123
lines changed

1 file changed

+85
-123
lines changed

‎src/backend/access/nbtree/nbtsearch.c

Lines changed: 85 additions & 123 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
*
88
*
99
* IDENTIFICATION
10-
* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.50 1999/07/1604:58:30 momjian Exp $
10+
* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.51 1999/07/1622:17:06 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -26,8 +26,6 @@
2626

2727
staticBTStack_bt_searchr(Relationrel,intkeysz,ScanKeyscankey,
2828
Buffer*bufP,BTStackstack_in);
29-
staticOffsetNumber_bt_firsteq(Relationrel,TupleDescitupdesc,Pagepage,
30-
Sizekeysz,ScanKeyscankey,OffsetNumberoffnum);
3129
staticint_bt_compare(Relationrel,TupleDescitupdesc,Pagepage,
3230
intkeysz,ScanKeyscankey,OffsetNumberoffnum);
3331
staticbool
@@ -368,7 +366,9 @@ _bt_skeycmp(Relation rel,
368366
*comparison for every key in the scankey. _bt_binsrch() returns
369367
*the OffsetNumber of the first matching key on the page, or the
370368
*OffsetNumber at which the matching key would appear if it were
371-
*on this page.
369+
*on this page. (NOTE: in particular, this means it is possible to
370+
*return a value 1 greater than the number of keys on the page, if
371+
*the scankey is > all keys on the page.)
372372
*
373373
*By the time this procedure is called, we're sure we're looking
374374
*at the right page -- don't need to walk right. _bt_binsrch() has
@@ -385,8 +385,8 @@ _bt_binsrch(Relation rel,
385385
Pagepage;
386386
BTPageOpaqueopaque;
387387
OffsetNumberlow,
388-
mid,
389388
high;
389+
boolhaveEq;
390390
intnatts=rel->rd_rel->relnatts;
391391
intresult;
392392

@@ -395,148 +395,112 @@ _bt_binsrch(Relation rel,
395395
opaque= (BTPageOpaque)PageGetSpecialPointer(page);
396396

397397
/* by convention, item 1 on any non-rightmost page is the high key */
398-
low=mid=P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY;
398+
low=P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY;
399399

400400
high=PageGetMaxOffsetNumber(page);
401401

402402
/*
403-
* Since for non-rightmost pages, the first item on the page is the
404-
* high key, there are two notions of emptiness. One is if nothing
405-
* appears on the page. The other is if nothing but the high key
406-
* does. The reason we test high <= low, rather than high == low, is
407-
* that after vacuuming there may be nothing *but* the high key on a
408-
* page. In that case, given the scheme above, low = 2 and high = 1.
403+
* If there are no keys on the page, return the first available slot.
404+
* Note this covers two cases: the page is really empty (no keys),
405+
* or it contains only a high key. The latter case is possible after
406+
* vacuuming.
409407
*/
410-
411-
if (PageIsEmpty(page))
408+
if (high<low)
412409
returnlow;
413-
if ((!P_RIGHTMOST(opaque)&&high <=low))
414-
{
415-
if (high<low||
416-
(srchtype==BT_DESCENT&& !(opaque->btpo_flags&BTP_LEAF)))
417-
returnlow;
418-
/* It's insertion and high == low == 2 */
419-
result=_bt_compare(rel,itupdesc,page,keysz,scankey,low);
420-
if (result>0)
421-
returnOffsetNumberNext(low);
422-
returnlow;
423-
}
424410

425-
while ((high-low)>1)
411+
/*
412+
* Binary search to find the first key on the page >= scan key.
413+
* Loop invariant: all slots before 'low' are < scan key, all slots
414+
* at or after 'high' are >= scan key. Also, haveEq is true if the
415+
* tuple at 'high' is == scan key.
416+
* We can fall out when high == low.
417+
*/
418+
high++;/* establish the loop invariant for high */
419+
haveEq= false;
420+
421+
while (high>low)
426422
{
427-
mid=low+ ((high-low) /2);
423+
OffsetNumbermid=low+ ((high-low) /2);
424+
/* We have low <= mid < high, so mid points at a real slot */
425+
428426
result=_bt_compare(rel,itupdesc,page,keysz,scankey,mid);
429427

430428
if (result>0)
431-
low=mid;
432-
elseif (result<0)
433-
high=mid-1;
429+
low=mid+1;
434430
else
435431
{
436-
mid=_bt_firsteq(rel,itupdesc,page,keysz,scankey,mid);
437-
438-
/*
439-
* NOTE for multi-column indices: we may do scan using keys
440-
* not for all attrs. But we handle duplicates using all attrs
441-
* in _bt_insert/_bt_spool code. And so while searching on
442-
* internal pages having number of attrs > keysize we want to
443-
* point at the last item < the scankey, not at the first item
444-
* = the scankey (!!!), and let _bt_moveright decide later
445-
* whether to move right or not (see comments and example
446-
* there). Note also that INSERTions are not affected by this
447-
* code (natts == keysz). - vadim 04/15/97
448-
*/
449-
if (natts==keysz||opaque->btpo_flags&BTP_LEAF)
450-
returnmid;
451-
low=P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY;
452-
if (mid==low)
453-
returnmid;
454-
returnOffsetNumberPrev(mid);
432+
high=mid;
433+
haveEq= (result==0);
455434
}
456435
}
457436

458-
/*
459-
* We terminated because the endpoints got too close together.There
460-
* are two cases to take care of.
437+
/*--------------------
438+
* At this point we have high == low, but be careful: they could point
439+
* past the last slot on the page. We also know that haveEq is true
440+
* if and only if there is an equal key (in which case high&low point
441+
* at the first equal key).
461442
*
462-
* For non-insertion searches on internal pages, we want to point at the
463-
* last key <, or first key =, the scankey on the page. This
464-
* guarantees that we'll descend the tree correctly. (NOTE comments
465-
* above for multi-column indices).
466-
*
467-
* For all other cases, we want to point at the first key >= the scankey
468-
* on the page. This guarantees that scans and insertions will happen
469-
* correctly.
443+
* On a leaf page, we always return the first key >= scan key
444+
* (which could be the last slot + 1).
445+
*--------------------
470446
*/
471447

472-
if (!(opaque->btpo_flags&BTP_LEAF)&&srchtype==BT_DESCENT)
473-
{/* We want the last key <, or first key
474-
* ==, the scan key. */
475-
result=_bt_compare(rel,itupdesc,page,keysz,scankey,high);
448+
if (opaque->btpo_flags&BTP_LEAF)
449+
returnlow;
476450

477-
if (result==0)
478-
{
479-
mid=_bt_firsteq(rel,itupdesc,page,keysz,scankey,high);
451+
/*--------------------
452+
* On a non-leaf page, there are special cases:
453+
*
454+
* For an insertion (srchtype != BT_DESCENT and natts == keysz)
455+
* always return first key >= scan key (which could be off the end).
456+
*
457+
* For a standard search (srchtype == BT_DESCENT and natts == keysz)
458+
* return the first equal key if one exists, else the last lesser key
459+
* if one exists, else the first slot on the page.
460+
*
461+
* For a partial-match search (srchtype == BT_DESCENT and natts < keysz)
462+
* return the last lesser key if one exists, else the first slot.
463+
*
464+
* Old comments:
465+
*For multi-column indices, we may scan using keys
466+
*not for all attrs. But we handle duplicates using all attrs
467+
*in _bt_insert/_bt_spool code. And so while searching on
468+
*internal pages having number of attrs > keysize we want to
469+
*point at the last item < the scankey, not at the first item
470+
*= the scankey (!!!), and let _bt_moveright decide later
471+
*whether to move right or not (see comments and example
472+
*there). Note also that INSERTions are not affected by this
473+
*code (since natts == keysz for inserts). - vadim 04/15/97
474+
*--------------------
475+
*/
480476

481-
/*
482-
* If natts > keysz we want last item < the scan key. See
483-
* comments above for multi-column indices.
484-
*/
485-
if (natts==keysz)
486-
returnmid;
487-
low=P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY;
488-
if (mid==low)
489-
returnmid;
490-
returnOffsetNumberPrev(mid);
491-
}
492-
elseif (result>0)
493-
returnhigh;
494-
else
495-
returnlow;
477+
if (haveEq)
478+
{
479+
/*
480+
* There is an equal key. We return either the first equal key
481+
* (which we just found), or the last lesser key.
482+
*
483+
* We need not check srchtype != BT_DESCENT here, since if that
484+
* is true then natts == keysz by assumption.
485+
*/
486+
if (natts==keysz)
487+
returnlow;/* return first equal key */
496488
}
497489
else
498-
/* we want the first key >= the scan key */
499490
{
500-
result=_bt_compare(rel,itupdesc,page,keysz,scankey,low);
501-
if (result <=0)
502-
returnlow;
503-
else
504-
{
505-
if (low==high)
506-
returnOffsetNumberNext(low);
507-
508-
result=_bt_compare(rel,itupdesc,page,keysz,scankey,high);
509-
if (result <=0)
510-
returnhigh;
511-
else
512-
returnOffsetNumberNext(high);
513-
}
491+
/*
492+
* There is no equal key. We return either the first greater key
493+
* (which we just found), or the last lesser key.
494+
*/
495+
if (srchtype!=BT_DESCENT)
496+
returnlow;/* return first greater key */
514497
}
515-
}
516498

517-
staticOffsetNumber
518-
_bt_firsteq(Relationrel,
519-
TupleDescitupdesc,
520-
Pagepage,
521-
Sizekeysz,
522-
ScanKeyscankey,
523-
OffsetNumberoffnum)
524-
{
525-
BTPageOpaqueopaque;
526-
OffsetNumberlimit;
527499

528-
opaque= (BTPageOpaque)PageGetSpecialPointer(page);
500+
if (low== (P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY))
501+
returnlow;/* there is no prior item */
529502

530-
/* skip the high key, if any */
531-
limit=P_RIGHTMOST(opaque) ?P_HIKEY :P_FIRSTKEY;
532-
533-
/* walk backwards looking for the first key in the chain of duplicates */
534-
while (offnum>limit
535-
&&_bt_compare(rel,itupdesc,page,
536-
keysz,scankey,OffsetNumberPrev(offnum))==0)
537-
offnum=OffsetNumberPrev(offnum);
538-
539-
returnoffnum;
503+
returnOffsetNumberPrev(low);
540504
}
541505

542506
/*
@@ -571,7 +535,6 @@ _bt_compare(Relation rel,
571535
{
572536
Datumdatum;
573537
BTItembtitem;
574-
ItemIditemid;
575538
IndexTupleitup;
576539
BTPageOpaqueopaque;
577540
ScanKeyentry;
@@ -589,12 +552,11 @@ _bt_compare(Relation rel,
589552
*/
590553

591554
opaque= (BTPageOpaque)PageGetSpecialPointer(page);
555+
592556
if (!(opaque->btpo_flags&BTP_LEAF)
593557
&&P_LEFTMOST(opaque)
594558
&&offnum==P_HIKEY)
595559
{
596-
itemid=PageGetItemId(page,offnum);
597-
598560
/*
599561
* we just have to believe that this will only be called with
600562
* offnum == P_HIKEY when P_HIKEY is the OffsetNumber of the first
@@ -621,7 +583,7 @@ _bt_compare(Relation rel,
621583
* on the page is greater than anything.
622584
*/
623585

624-
if (_bt_skeycmp(rel,keysz,scankey,page,itemid,
586+
if (_bt_skeycmp(rel,keysz,scankey,page,PageGetItemId(page,offnum),
625587
BTEqualStrategyNumber))
626588
return0;
627589
return1;

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